Thin thermoplastic resin films have been used at restaurants, food shops or home when food is stored or heated in a microwave oven. Among them, a wrap film made of a vinylidene-chloride-based copolymer resin is used frequently as a food packaging wrap film, because it is equipped with excellent properties including moisture resistance, oxygen gas barrier properties, heat resistance, clinging property to a container and transparency.
In recent years, various food packaging wrap films composed mainly of a polyolefin-based resin have been proposed. Examples of such films include polyethylene-based resins, polypropylene-based resins, and poly-4-methylpentene-1 resins. The surface of these films hardly has a clinging property so that when they are used, for example, as a food packaging film, they do not cling to a container sufficiently, which is a fatal defect for them. A number of polyolefin-based films mixed with various additives or another resin, or laminated with another resin have been proposed in order to satisfy such a desired performance. They are however inferior in practical usability, because not only the clinging property to a container but also a film-to-film clinging property is heightened, which deteriorates pulling-out ease from a dispenser box.
With a view to overcoming the above-described various problems, various proposals have been made as to the clinging property of a wrap film. JP-A-10-202806 proposes a self-cohesive wrap film comprising a core layer of a polypropylene-based resin and a surface layer containing a surfactant as an adhesive. However, it is difficult to attain a high clinging property by this technique. Moreover, when a food material having a high water content is wrapped with the wrap film and heated in a microwave oven, it raises a problem of bubbling of the surfactant on the wrap film surface by the action of water.
Heightening of the clinging property leads to necessity of a high pulling-out force, while lowering of the pulling-out force leads to a deterioration in the clinging property. An increase in a modulus of elasticity, which is an index of stiffness, worsens stretching property. Thus, characteristics necessary for a wrap film tend to contradict each other. It is therefore a very difficult to problem to maintain the balance between these characteristics.
For example, JP-A-2002-46238 proposes a multilayer film which comprises a core layer comprising a resin having a barrier property, and a surface layer comprising a resin composition containing an additive having a clinging property. However, since additives to attain the clinging property have a low molecular weight or a low glass transition point, they show a phenomenon called “bleed in” and transfer in the film. As a result, in spite that the film exhibits good balance of clinging property and pulling-out ease just after film formation, the clinging property and pulling-out ease happen to be deteriorated with the passage of time owing to the transfer of the additive inwards from the surface layer.
A wrap film using a poly(4-methylpentene-1) resin has been proposed frequently, for example, in JP-A-2001-121660, because the resin has excellent heat resistance. Since the poly(4-methylpentene-1) resin is a material of high rigidity, a large amount of a plasticizer must be added to satisfy the level of flexibility which a wrap film is required to have. If it is added, however, the heat resistance or low tensile elongation at break which the resin essentially has is impaired.